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A Emergence of spin 2 particles from lowers spin particles

  1. Oct 2, 2016 #1
    I was referring a lecture series by Professor Hong Liu. In his first lecture on Emergence of gravity he says, "massless spin-2 particles (gravitons) can arise as bound states in a theory of massless spin-1 (photons, gluons) and spin- 1 particles (protons, electrons)".

    What does it exactly means?
    https://ocw.mit.edu/courses/physics...-fall-2014/lecture-notes/MIT8_821S15_Lec1.pdf (Link to lecture 1)
     
  2. jcsd
  3. Oct 2, 2016 #2

    ohwilleke

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    You mean to say "and massive spin - 1/2 particles (protons, electrons)". It is quite odd to include "protons" which are not fundamental, however, in the discussion of photons, gluons and electrons which are fundamental.
     
  4. Oct 10, 2016 #3
    Yes, the massive spin 1/2 particles are also included.
     
  5. Oct 10, 2016 #4

    atyy

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    That is the opposite of what he says. You have misread that paragraph.
     
  6. Oct 10, 2016 #5

    Chronos

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    Professor Liu's referenced lecture note [page 2, para 2] says "...massless spin-1 (photons, gluons) and spin-1/2 particles (protons, electron)...".
     
  7. Oct 10, 2016 #6

    Haelfix

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    The paragraph is a bit hard to read, but Atyy is correct. Historically people thought about deriving Spin2 gravity from bound states of lower spin particles. The Weinberg-Witten theorem, which is what the lecture is about, was a direct refutation of this line of thought.

    Interestingly, since holography became popular, it showed a failure mode of the theorem. HOlography allows gravity to emerge from a theory of 'gluons' (various SU(N) theories) living on the boundary, and critically uses a different metric so is allowed to circumvent one of the assumptions of the theory.
     
  8. Oct 10, 2016 #7
    is holography saying that gluons, which are self-interacting, can give rise to gravity, such as spin-2 glueballs?
     
  9. Oct 11, 2016 #8

    ohwilleke

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    No. It is saying that a theorem on the four-dimensional boundary of a space (which itself has only three dimensions) which is part of the same class of theories as QCD can explain 4D gravity, since gravity within a space can be fully determined by its boundary conditions and hence has fewer degrees of freedom than one would naively expect. Spin-2 glueballs have nothing to do with gravity.
     
  10. Oct 11, 2016 #9
    Can the boundary of a space have more dimensions than the space itself?
     
  11. Oct 12, 2016 #10

    ohwilleke

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    Not under any reasonable set of assumptions. Generally, the boundary will have one less dimension that the space it contains.
     
  12. Oct 12, 2016 #11

    ohwilleke

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    A similar idea is expressed in this (open access) paper: http://www.sciencedirect.com/science/article/pii/S0370269314007904
     
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